Riot police in Chile have deployed water cannons and tear gas against demonstrators who were lobbing rocks and street signs, after a mostly-peaceful march against a contentious university reform and corruption allegations ended in violent clashes.

Organizers say that 150,000 students filled the streets of Santiago on Thursday to demand free higher education, and challenge Socialist party president Michelle Bachelet over recent corruption allegations. Authorities estimated the number to have stood at around 20,000 people.

As the peaceful march, organized mostly by opposition parties, came to a close, a smaller group of radical protesters began to attack and pelt objects at the armed police, reportedly injuring one officer. Riot police pushed back, using water cannon and tear gas, as demonstrators banged on the windows of armored vans. At least four protesters were detained.